[PATCH 3.2 196/221] macvtap: limit head length of skb allocated

[Ben Hutchings]

3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jason Wang <jasowang@xxxxxxxxxx>

commit 16a3fa28630331e28208872fa5341ce210b901c7 upstream.

We currently use hdr_len as a hint of head length which is advertised by
guest. But when guest advertise a very big value, it can lead to an 64K+
allocating of kmalloc() which has a very high possibility of failure when host
memory is fragmented or under heavy stress. The huge hdr_len also reduce the
effect of zerocopy or even disable if a gso skb is linearized in guest.

To solves those issues, this patch introduces an upper limit (PAGE_SIZE) of the
head, which guarantees an order 0 allocation each time.

Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/net/macvtap.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -672,6 +672,7 @@ static ssize_t macvtap_get_user(struct m
 				const struct iovec *iv, unsigned long total_len,
 				size_t count, int noblock)
 {
+	int good_linear = SKB_MAX_HEAD(NET_IP_ALIGN);
 	struct sk_buff *skb;
 	struct macvlan_dev *vlan;
 	unsigned long len = total_len;
@@ -714,6 +715,8 @@ static ssize_t macvtap_get_user(struct m
 
 	if (m && m->msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
 		copylen = vnet_hdr.hdr_len ? vnet_hdr.hdr_len : GOODCOPY_LEN;
+		if (copylen > good_linear)
+			copylen = good_linear;
 		linear = copylen;
 		if (iov_pages(iv, vnet_hdr_len + copylen, count)
 		    <= MAX_SKB_FRAGS)
@@ -722,7 +725,10 @@ static ssize_t macvtap_get_user(struct m
 
 	if (!zerocopy) {
 		copylen = len;
-		linear = vnet_hdr.hdr_len;
+		if (vnet_hdr.hdr_len > good_linear)
+			linear = good_linear;
+		else
+			linear = vnet_hdr.hdr_len;
 	}
 
 	skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, copylen,

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